Self-ligand stimulation is essential for T cell development in the thymus and the induction of central tolerance. The importance of peripheral self-ligand stimulation, however, is only beginning to be appreciated. The preliminary data in this proposal indicates that CD4 T cells deprived of peripheral self-ligand stimulation develop a progressive defect in their ability to physically interact with dendritic cells (DCs) bearing cognate antigen. Thus, results in suboptimal T cell activation as measured by CD69 expression and in a profound defect in CD4 T cell proliferation. The central hypotheses, to be tested, are that self-ligand stimulation is essential in conditioning T cell receptivity to interaction with antigen-presenting cells (APCs) and that self-ligand deprived T cells, incapable of maintaining sustained contact with antigen-bearing APCs, follow an altered developmental pathway resulting in immunological tolerance. Tolerance will be tested using unique transplantation models and flow cytometry staining of in vivo self-ligand deprived CD4 T cells. Specifically, we will test whether self-ligand deprivation (1) impairs T cell motility within secondary lymphoid tissue and the ability of the T cell to find cognate antigen-bearing APCs, (2) impairs the ability of T cells to form transient conjugates with APCs, scan for antigens and form immunological synapses, facilitating TCR signaling, and/or (3) impairs the ability of T cells to form long-lived antigen-dependent clusters with DCs, facilitating T cell differentiation. In addition, we will explore the mechanism(s) underlying these impaired T cell-DC interactions. It is proposed that the inability of T cells to make contact, signal through the TCR and firmly adhere to cognate antigen-bearing DCs will result in inefficient activation and thus, tolerance. Understanding the mechanism of T cells interaction with cognate DCs is clearly important. It is argued that the ability to modulate T cell responses via self-ligand deprivation can have a strong impact on transplantation and autoimmunity. This proposal considers that interrupting T cell contact with peripheral self-peptide/major histocompatibility complex (MHC) molecules (self-ligands) prior to and at the time of antigen exposure may provide novel therapeutic strategies to achieve antigen-specific T cell unresponsiveness. [unreadable] [unreadable] [unreadable]